President Bush himself has not directly responded to Mr Annan's comments.

But while campaigning in Minnesota for November's presidential election, he reiterated the US position that Security Council resolution 1441, passed in November 2002, gave the invasion legal force.

"The United Nations looked at the same intelligence I looked at," AFP news agency quoted Mr Bush as saying. "They concluded Saddam Hussein was a threat. They voted by 15-0 in the UN Security Council for Saddam Hussein to disclose, disarm or face serious consequences."

Mr Annan's criticism has particular political resonance in the US, where the Bush administration's performance on Iraq is a key issue in the election, the BBC's Jill McGivering reports from Washington.

Mr Annan's disapproval of the US-led action was already well known. But this is the first time he has used the word "illegal", our correspondent says.